The present invention generally relates to semiconductor lasers, and more particularly, to a laser structure and method for manufacturing a single transverse mode Vertical-Cavity Surface-Emitting Laser (VCSEL).
Various schemes for producing semiconductor diode lasers are known in the art. Conventional edge-emitting diode lasers comprise an optical cavity that is parallel to the surface of the wafer from which many laser dice (or chips) are produced from a wafer by sawing or cleaving. Laser radiation is extracted from the side of the die in an edge-emitting laser. Surface-emitting lasers, in contrast, emit radiation perpendicular to the semiconductor substrate plane, from the top or bottom of the die. A Vertical-Cavity Surface-Emitting Laser (VCSEL) is a surface-emitting laser having mirrors disposed parallel to the wafer surfaces that form and enclose an optical cavity between them. Recently there has been an increased interest in VCSELs because of their smaller size, higher performance, and ease of manufacturability when compared to edge-emitting lasers. VCSELs have proven useful for multi-mode operation, in particular, and for use in modern high speed, short wavelength communication systems.
VCSELs usually have a substrate upon which a first mirror stack and second mirror stack is disposed, with a quantum well active region therebetween. Gain per pass is much lower with a VCSEL than an edge-emitting laser, which necessitates better mirror reflectivity. For this reason, the mirror stacks in a VCSEL typically comprise a plurality of Distributed Bragg Reflector (DBR) mirrors, which may have a reflectivity of 99.7% or higher. An electrical contact is positioned on the second mirror stack, and another contact is provided at the opposite end in contact with the substrate. When an electrical current is induced to flow between the two contacts, lasing is induced from the active region and emits through either the top or bottom surface of the VCSEL.
VCSELs may be broadly categorized into multi-transverse mode and single transverse mode, each category being advantageous in different circumstances. A goal in manufacturing single-mode VCSELs is to assume single-mode behavior over all operating conditions, without compromising other performance characteristics. Generally, the active regions of single transverse mode VCSELs require small lateral dimensions, which tend to increase the series resistance and beam divergence angle. Furthermore, a device that is single-mode at one operating condition can become multi-mode at another operating condition, an effect that dramatically increases the spectral width and the beam divergence of the emitted radiation of the VCSEL.
Manufacturing a VCSEL with good mode control and high performance characteristics poses a challenge. It is difficult to manufacture VCSELs that efficiently operate in the lowest order mode (single mode). Most prior art VCSELs tend to lase in higher-order transverse modes, whereas single transverse mode lasing is preferred for some applications, such as sensors.
In the June 1991 issue of IEEE Journal of Quantum Electronics, Vol. 27, No. 6, in an article entitled, xe2x80x9cVertical-Cavity Surface-Emitting Lasers: Design, Growth, Fabrication, Characterization,xe2x80x9d Jewell et al. discuss design issues, molecular beam epitaxial (MBE) growth, fabrication and lasing characteristics of VCSELs in general.
In the August, 1998 publication of IEEE Photonics Technology Letters, Vol. 10, No. 8, in article entitled xe2x80x9cSingle-Mode Operation in an Antiguided Vertical-Cavity Surface Emitting Laser Using a Low-temperature Grown AlGaAs Dielectric Aperture,xe2x80x9d Oh et al. discuss using a low-temperature growth of a highly resistive AlGaAs dielectric aperture. A reduced regrowth temperature is required to obtain smooth boundaries over the aperture perimeter.
U.S. Pat. No. 5,903,588, xe2x80x9cLaser with a Selectively Changed Current Confining Layer,xe2x80x9d which issued to Guenter et al. on May 11, 1999, discloses a laser structure with two current confirming layers of a material that is subject to oxidation in the presence of an oxidizing agent. Unoxidized layer portions are surrounded by oxidized and electrically resistive ports in order to direct current from one electrical contact pad by passing through a pre-selected portion of an active region of the laser.
In the June 1995 issue of the IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2, in an article entitled xe2x80x9cSingle-Mode, Passive Antiguide Vertical Cavity Surface Emitting Laser,xe2x80x9d We et al. discuss using a passive antiguide region surrounding the active region to achieve a single stable mode at high currents. This design is disadvantageous because a mesa structure is formed, and then material must be regrown around the side, while maintaining single crystal characteristics.
In the October 1997 issue of the IEEE Photonics Technology Letters, Vol. 9, No. 10, in an article entitled xe2x80x9cEfficient Single-Mode Oxide-Confined GaAs VCSELs Emitting in the 850-nm Wavelength Regimexe2x80x9d, Grabherr et al. disclose a single mode oxide-confined VCSEL. An oxidized current aperture is placed adjacent the active region of the VCSEL.
In the Apr. 1, 1998 publication of the Journal of Applied Physics, Vol. 83, No. 7, in an article entitled xe2x80x9cEffect of Reflectivity at the Interface of Oxide Layer on Transverse Mode Control in Oxide Confined Vertical-Cavity Surface-Emitting Lasers,xe2x80x9d Huang demonstrates transverse mode control by modeling the dielectric aperture as a uniform waveguide and an extra reflectivity at the oxide layer. Huang shows that replacing the first layer of the DBR with a xc2xe wavelength layer immediately adjacent the optical cavity, and inserting an oxide layer inside the xc2xe wave layer, results in a low refractive index step and lower threshold current.
Usually, to manufacture a VCSEL, a relatively large current aperture size is required to achieve a low series resistance and high power output. A problem with a large current aperture is that higher order lasing modes are introduced so that single mode lasing only occurs just above threshold, if at all. Manufacturing a VCSEL with a smaller current aperture to more reliably obtain single mode behavior causes multiple problems: the series resistance and beam divergence angle become large, and the attainable power becomes small. Anti-guide structures of the prior art prevent some of these disadvantages, but suffer from manufacturing difficulties, particularly in requiring an interruption in epitaxial growth, a patterning step, and subsequent additional epitaxy. Other large single mode VCSELs require multi-step MBE or MBE/MOCVD combinations to manufacture, creating alignment and yield problems.
What is needed in the art is a VCSEL with improved mode control and ease of manufacturing, with as large as possible an aperture, to minimize the series resistance and beam divergence.
The present invention achieves technical advantages as a VCSEL designed for single mode operation whereby a phase shifting region, which creates a coupled cavity, is disposed within one mirror stack. The coupled cavity decreases reflectance as seen from the active region. This phase shifting region is disposed nominally outside the optical aperture. The resultant reflectance increases losses for higher order modes relative to the fundamental mode because higher order modes have a larger spatial extent. Centering the phase shifting region at a node of the optical electric field of the VCSEL maximizes the losses for higher order modes relative to the fundamental mode. In addition to the enhanced losses the present invention also creates an antiguide.
According to a first embodiment, disclosed is a laser structure adapted to lase at a wavelength lambda. The laser structure includes a first mirror stack, an active region disposed on the first mirror stack, and a second mirror stack disposed on the active region. The second mirror stack includes a plurality of mirror layer pairs with a resonant layer having a phase shifting region about a current aperture disposed therein outside of the quantum well layers of the active region. The phase shifting region creates a coupled cavity positioned outside the optical aperture and is positioned at least one mirror period above the active region. The optical thickness of the phase shifting region is preferably an odd integral multiple of one-fourth lambda different from the parallel optical path length inside the current aperture. At the operating wavelength the reflectance of the second mirror stack outside the optical aperture is reduced with the present invention.
Also disclosed is a laser structure adapted to lase at a wavelength lambda, including a first mirror stack, an active region disposed on the first mirror stack, and a second mirror stack disposed on the active region. The second mirror stack includes a plurality of semiconductor layers, and has a resonant layer with a phase shifting region disposed therein nominally outside an optical aperture. The phase shifting region creates a coupled cavity positioned vertically near the active layers, but outside the optical aperture. At least one of the plurality of semiconductor layers is disposed between the phase shifting region and the active region, and the phase shifting region has a lower index of refraction than the adjacent semiconductor layers proximate the active region. The thickness of the phase shifting region is a function of one-fourth lambda and a function of the difference between the index of refraction of the adjacent semiconductor layer and the index of refraction of the phase shifting region.
Also disclosed is a vertical cavity surface emitting laser (VCSEL) adapted to lase at a wavelength lambda. The VCSEL has a first semiconductor mirror stack, an active region disposed on the first semiconductor mirror stack, and a second semiconductor mirror stack disposed on the active region. The second semiconductor mirror stack has a plurality of semiconductor mirror layer pairs, with a phase shifting region disposed therein about a current aperture. The phase shifting creates a coupled cavity positioned near but spaced from the active region. The phase shifting region is positioned such that reflectance of the second mirror stack is reduced nominally outside the optical aperture, and has a thickness defined by the formula:   d  =                    (                  1          +                      2            ⁢            j                          )            *      λ                      (                              n            semi                    -                      n            oxide                          )            *      4      
wherein d is the phase shifting region thickness, j is an integer, nsemi is the index of refraction of the horizontally adjacent semiconductor layer to the phase shifting region at the lasing wavelength xcex, and noxide is the index of refraction of the phase shifting region at the lasing wavelength xcex.
Further disclosed is a method for manufacturing a vertical-cavity surface-emitting laser (VCSEL). The method includes the steps of providing a first semiconductor mirror stack, forming an active region upon the first semiconductor mirror stack, and forming a first portion of a second semiconductor mirror stack above the active region. The first portion of the second semiconductor mirror stack has at least one mirror layer pair. Also included are the steps of forming a phase shifting region disposed about a current aperture above the first portion of the second semiconductor mirror stack, the phase shifting region and the mirror regions above and below the phase shifting region forming a coupled cavity, and thereafter, a second portion of a second semiconductor mirror stack is formed above the phase shifting region.
Advantages of the invention include reducing reflectance of the second semiconductor mirror stack in the coupled region, which causes losses for higher order modes, enhancing single mode performance of the laser. The phase shifting region of the present invention may be created by forming an oxide layer using single step MOCVD, combined with an oxidation step, improving manufacturability and increasing yields.